The dealer would approach guests and have a chance to make a deal, which would trigger them to both walk to a more remote area of the park to complete the transaction. The guest would appear intoxicated in some other way than the drunks appear, and would then have an elevated risk of drowning, slipping, littering, peeing in pools, injuring themselves by jumping too far off of diving boards, etc. At this point, both the guest and dealer would be nasty guests and give a bonus for catching them. The dealer would be recognizable through approaching guests and eventually walking together with one, and as they emerge after the deal with a visible intoxicated guest.
Catching the dealer could grant some choices:
-Kick them out using tazer, pushing, baseball bat, etc., granting bonus money for catching them
-Allow a drug dealing ring to exist in the park for a cut of the profits. The ring would last for some set number of days, and each day would have a chance to trigger a DEA raid, resulting in forfeiture of profits from the ring plus additional fines. The DEA raid chance would increase based on the number of injured guests or concurrent intoxicated guests, so catching the intoxicated ones is still incentivized.
EDIT: Maintain game maturity level
Maybe it should be more tongue-in-cheek and keep the same mechanics. The shady vendor who sells smuggled snacks, questionable lemonade, mystery gummies, bootleg energy drinks instead of drugs. It could keep all the subtext and stay PG-13.